Multi-antenna techniques can significantly increase the data rates and reliability of a wireless communication system. The performance is in particular improved if both the transmitter and the receiver are equipped with multiple antennas, which results in a multiple-input multiple-output (MIMO) communication channel. Such systems and/or related techniques are commonly referred to as MIMO.
The 3GPP long term evolution (LTE) standard is currently evolving with enhanced MIMO support. A core component in LTE is the support of MIMO antenna deployments and MIMO related techniques. Currently LTE-Advanced supports an 8-layer spatial multiplexing mode for up to 16 transmit antennas with channel dependent precoding. The spatial multiplexing mode is aimed for high data rates in favorable channel conditions. An illustration of the spatial multiplexing operation by a precoding matrix 2 is provided in FIG. 1.
As seen, the information carrying symbol vectors from layers 1-r 4 is multiplied by an NT×r precoder matrix W, 2 which serves to distribute the transmit energy in a subspace of the NT (corresponding to NT antenna ports) dimensional vector space to produce signals to be inverse Fourier transformed 6.
The precoder matrix 2 is typically selected from a codebook of possible precoder matrices, and typically indicated by means of a precoder matrix indicator (PMI), which specifies a unique precoder matrix in the codebook for a given number of symbol streams. The r symbols in s each correspond to a layer and r is referred to as the transmission rank. In this way, spatial multiplexing is achieved since multiple symbols can be transmitted simultaneously over the same time/frequency resource element (TFRE). The number of symbols r is typically adapted to suit the current channel properties.
LTE uses OFDM in the downlink (and DFT preceded OFDM in the uplink) and hence the received NR×1 vector yn for a certain TFRE on subcarrier n (or alternatively data TFRE number n) is thus modeled byyn=HnWsn+en  Equation 1
where en is a noise/interference vector obtained as realizations of a random process. The precoder W can be a wideband precoder, which is constant over frequency, or frequency selective.
The precoder matrix W 2 is often chosen to match the characteristics of the NR×NT MIMO channel matrix Hn, resulting in so-called channel dependent preceding. This is also commonly referred to as closed-loop precoding and essentially strives for focusing the transmit energy into a subspace which is strong in the sense of conveying much of the transmitted energy to the wireless device. In addition, the precoder matrix may also be selected to strive for orthogonalizing the channel, meaning that after proper linear equalization at the wireless device, the inter-layer interference is reduced.
One example method for a wireless device to select a precoder matrix W can be to select the Wk that maximizes the Frobenius norm of the hypothesized equivalent channel:
                              max          k                ⁢                                                                                          H                  ^                                n                            ⁢                              W                k                                                          F          2                                    Equation        ⁢                                  ⁢        2            
Where
Ĥn s a channel estimate, possibly derived from CSI-RS as described later.
Wk is a hypothesized precoder matrix with index k.
ĤnWk is the hypothesized equivalent channel.
In closed-loop preceding for the LTE downlink, the wireless device transmits, based on channel measurements in the forward link (downlink), recommendations to the base station, e.g., eNodeB (eNB) of a suitable precoder to use. The base station configures the wireless device to provide feedback according to the wireless device's transmission mode, and may transmit CSI-RS and configure the wireless device to use measurements of CSI-RS to feedback recommended preceding matrices that the wireless device selects from a codebook. A single precoder that is supposed to cover a large bandwidth (wideband precoding) may be fed back. It may also be beneficial to match the frequency variations of the channel and instead feedback a frequency-selective precoding report, e.g., several precoders, one per subband. This is an example of the more general case of channel state information (CSI) feedback, which also encompasses feeding back other information that recommended precoders to assist the NodeB in subsequent transmissions to the wireless device. Such other information may include channel quality indicators (CQIs) as well as transmission rank indicator (RI).
In LTE, the format of the CSI reports is specified in detail and may contain CQI (Channel-Quality Information), Rank Indicator (RI), and Precoding Matrix Indicator (PMI). The reports can be wideband (i.e. applicable to the whole bandwidth) or subbands (i.e. applicable to part of the bandwidth). They can be configured by a radio resource control (RRC) message to be sent periodically or in an aperiodic manner triggered by a DCI sent from the eNB to a WD. The quality and reliability of the CSI are crucial for the eNB in order to make the best possible scheduling decisions for the upcoming DL transmissions.
An aperiodic CSI request is indicated in the CSI request field in DCI format 0 or DCI format 4. The number of hits in the field varies from 1 bit to 3 bits, depending on WD configuration. For example, for WDs configured with 1 to 5 carriers (or cells) and/or multiple CSI-RS processes, 2 bits are used, and for WDs configured with more than 5 carriers, 3 bits are used. In case a WD is configured with a single carrier (i.e. serving cell c) and 2 sets of CSI-RS processes, the CSI request field is shown in Table 1. If a WD is configured with a single carrier and a single or no CSI process, a single bit is used. The concept of CSI process was introduced in LTE Rel-11, where a CSI process is defined as a configuration of a channel measurement resource and an interference measurement resource and up to four CSI processes can be configured for a WD.
TABLE 1Value of CSIrequest fieldDescription‘00’No aperiodic CSI report is triggered‘01’Aperiodic CSI report is triggered for a set of CSIprocess(es) configured by higher layers for serving cellc‘10’Aperiodic CSI report is triggered for a 1st set of CSIprocess(es) configured by higher layers‘11’Aperiodic CSI report is triggered for a 2nd set of CSIprocess(es) configured by higher layers
With regard to CSI feedback, a subband is defined as a number of adjacent PRB pairs. In LTE, the subband size (i.e., the number of adjacent PRB pairs) depends on the system bandwidth, whether CSI reporting is configured to be periodic or aperiodic, and feedback type (i.e., whether higher layer configured feedback or wireless device-selected subband feedback is configured). An example illustrating the difference between subband and wideband is shown in FIG. 2. In the example, the subband consists of 6 adjacent PRBs. Note that only 2 subbands are shown in FIG. 2 for simplicity of illustration. Generally, all the PRB pairs in the system bandwidth are divided into different subbands where each subband consists of a fixed number of PRB pairs.
In contrast, the wideband CSI feedback involves all the PRB pairs in the system bandwidth. As mentioned above, a wireless device may feedback a single precoder that takes into account the measurements from all PRB pairs in the system bandwidth if it is configured to report wideband PMI by the base station. Alternatively, if the wireless device is configured to report subband PMI, a wireless device may feedback multiple precoders with one precoder per subband. In addition, to the subband precoders, the wireless device may also feedback the wideband. PMI.
In LTE, two types of subband feedback types are possible for PUSCH CSI reporting: (1) higher layer configured subband feedback and (2) wireless device selected subband feedback. With higher layer configured subband feedback, the wireless device may feedback PMI and/or CQI for each of the subbands. The subband size in terms of the number of PRB pairs for higher layer configured subband feedback is a function of system bandwidth and is listed in Table 2. With wireless device selected subband feedback, the wireless device only feeds back PMI and/or CQI for a selected number of subbands out of all the subbands in the system bandwidth. The subband size in terms of the number of PRB pairs and the number of subbands to be fed back are a function of the system bandwidth and are listed in Table 3.
TABLE 2System BandwidthSubband SizeNRB(ksub)6-7NA 8-10411-26427-636 64-1108
TABLE 3System BandwidthSubband SizeNumber ofNRBDLk (RBs)Subbands6-7NANA 8-102111-262327-6335 64-11046
Given the CST feedback from the wireless device, the base station determines the transmission parameters it wishes to use to transmit to the wireless device, including the precoding matrix, transmission rank, and modulation and coding state (MCS). These transmission parameters may differ from the recommendations the wireless device makes. Therefore, a rank indicator and MCS may be signaled in downlink control information (DCI), and the precoding matrix can be signaled in DCI or the base station can transmit a demodulation reference signal from which the equivalent channel can be measured. The transmission rank, and thus the number of spatially multiplexed layers, is reflected in the number of columns of the precoder W. For efficient performance, it is important that a transmission rank that matches the channel properties is selected.
In LTE Release-10, a new reference signal was introduced to estimate downlink channel state information reference signals, (CSI-RS). The CSI-RS provides several advantages over basing the CSI feedback on the common reference signals (CRS) which were used for that purpose in Releases 8-9. First, the CSI-RS is not used for demodulation of the data signal, and thus does not require the same density (i.e., the overhead of the CSI-RS is substantially less). Second, CSI-RS provides a much more flexible means to configure CSI feedback measurements (e.g., which CSI-RS resource to measure on can be configured in a wireless device specific manner).
By measuring a CSI-RS transmitted from the base station, a wireless device can estimate the effective channel the CSI-RS is traversing including the radio propagation channel and antenna gains. In more mathematical rigor this implies that if a known CSI-RS signal x is transmitted, a wireless device can estimate the coupling between the transmitted signal and the received signal (i.e., the effective channel). Hence if no virtualization is performed in the transmission, the received signal y can be expressed asy=Hx+e  Equation 3
and the wireless device can estimate the effective channel H.
Up to eight CSI-RS ports can be configured in LIE Rel-10, that is, the wireless device can estimate the channel from up to eight transmit antenna ports. In LTE Release 13, the number of CSI-RS ports that can be configured is extended to up to sixteen ports. In LTE Release 14, supporting up to 32 CSI-RS ports is under consideration
Related to CSI-RS is the concept of zero-power CSI-RS resources (also known as muted CSI-RS) that are configured just as regular CSI-RS resources, so that a wireless device knows that the data transmission is mapped around those resources. The intent of the zero-power CSI-RS resources is to enable the network to mute the transmission on the corresponding resources in order to boost the signal-to-interference-plus-noise ratio (SINR) of a corresponding non-zero power CSI-RS, possibly transmitted in a neighbor cell/transmission point. For Rel-11 of LTE a special zero-power CSI-RS was introduced that a wireless device is mandated to use for measuring interference plus noise. A wireless device can assume that the serving evolved node B (eNB) is not transmitting on the zero-power CSI-RS resource, and the received power can therefore be used as a measure of the interference plus noise.
Based on a specified CSI-RS resource and on an interference measurement configuration (e.g., a zero-power CSI-RS resource), the wireless device can estimate the effective channel and noise plus interference, and consequently also determine the rank, preceding matrix, and MCS to recommend to best match the particular channel.
Existing solutions for MU-MIMO based on implicit CSI reports with DFT-based precoders have problems with accurately estimating and reducing the interference between co-scheduled users, leading to poor MU-MIMO performance.
Multi-beam precoder schemes may lead to better MU-MIMO performance, but at the cost of increased CSI feedback overhead and wireless device precoder search complexity. It is an open problem of how an efficient multi-beam precoder codebook that results in good MU-MIMO performance but low feedback overhead should be constructed, as well as how the CSI feedback should be derived by the wireless device.